In the art of aircraft fuel control systems, a common approach of metering fuel to gas turbine engines has included the use of a positive displacement pump, commonly a gear pump driven by the engine gear box, as the main fuel pump. In these prior systems, the pump outputs fuel flow in excess of engine burn requirements to ensure that sufficient fuel is always delivered to the engine. The output fuel flow is then metered by a selectively positioned fuel metering valve that restricts flow through the manifold to provide a selected flow rate in the manifold. Excess fuel is recirculated through a large fuel bypass from the manifold downstream of the pump back to the pump inlet.
A significant drawback with positive displacement pump fuel metering systems occurs during operation of the aircraft at low fuel consumption and high engine speed situations, such as when the aircraft is at idle descent from a high altitude. Under such conditions, the pump outputs a flow of fuel that is many times the metered flow needed by the engine combustion chamber. Therefore, much of the fuel is forced to recirculate through the fuel bypass which undesirably increases the temperature of the fuel. The fuel temperature increase from such high bypass flows causes various heat management problems for the aircraft and represents lost horsepower. These heat management problems have required aircraft to use larger, heavier and costlier heat exchangers and other heat management systems.
To overcome these heat management problems associated with positive displacement fuel metering systems, some aircraft fuel metering systems may include variable displacement pumps. Variable displacement pumps can reduce the large recirculated bypass flow associated with positive displacement pump fuel metering systems. To achieve a reduction in bypass flow, the displacement of the variable displacement pump is adjusted so that the pump output approximately equals the metered flow which is set by a selectively positioned fuel metering valve in the downstream manifold.
One significant disadvantage with prior variable displacement pumps in fuel metering systems is that of slower dynamic response. This is because the displacement of the pump is set by a servo actuator which in turn is driven by a portion of the pumped fuel flow in the downstream manifold. To achieve sufficient control over variable displacement pumps, the servo actuator needs a fairly large amount of displacement flow to overcome the friction forces within the variable displacement pump. This means that for sudden demands to increase pump output, a substantial amount of fuel is removed from the downstream manifold to drive the servo actuator and thereby change pump displacement accordingly. This causes a momentary dip in metered flow and may cause the metered flow to actually go the wrong direction after an initial step increase for the demanded metered flow.